(Vox) – By far one of the most important impacts of global warming in the coming decades will be sea-level rise. As the Earth heats up and ice sheets in Greenland and Antarctica melt, ocean levels will creep upward, flooding coastal cities and forcing large-scale relocations around the world.

But there’s a maddening asterisk here: We still don’t know exactly how high oceans will rise this century. Studies have suggested it could be anywhere from 2 to 6 feet, globally — with newer evidence leaning toward the higher end, depending on how rapidly parts of the large ice sheet in West Antarctica collapse. Worse, climate scientists probably won’t be able to pin down an exact number anytime soon, because getting a handle on ice-sheet dynamics is inherently tricky.

That’s not reason for complacency, though. It actually makes preparation more difficult, because coastal cities will have to start mounting defenses in the face of considerable uncertainty. This map, for instance, shows how different levels of sea-level rise could put different parts of New York City underwater.

That means, as climate scientists Michael Oppenheimer and Richard Alley explain in a new paper in Science, that coastal areas will have to learn to master the art of flexibility — developing sea walls and other defenses that can evolve over time — and be ready for a wide array of plausible outcomes. Meanwhile, scientists themselves need to get much better at conveying the “deep uncertainties” around ice sheets and sea levels.

“The response can’t just be to wait until the science clears up,” says Oppenheimer, a climate and geosciences expert at Princeton University. “Because it’s unlikely we’re going to get a sharp answer anytime soon. And if policymakers sit around waiting for a definitive answer, they could find it’s too late to avoid disastrous levels of change.” [more]

ABSTRACT: Recent estimates suggest that global mean sea level rise could exceed 2 m by 2100. These projections are higher than previous ones and are based on the latest understanding of how the Antarctic Ice Sheet has behaved in the past and how sensitive it is to future climate change. They pose a challenge for scientists and policy-makers alike, requiring far-reaching decisions about coastal policies to be made based on rapidly evolving projections with large, persistent uncertainties. An effective approach to managing coastal risk should couple research priorities to policy needs, enabling judicious decision-making while focusing research on key questions